Class

The idea of classes goes back long before Plato. The platonic solids were classes, instances of which could be seen in the world. The platonic sphere was absolutely perfect but insubstantial. The spheres around us we could touch, but they were all imperfect in some way.

Object-oriented programming picked up on this idea by way of later western philosophers, dividing programs into classes, which are general descriptions of a whole set of similar things, and objects, which are the things themselves.

Classes are important for communication because they describe, potentially, many specific things. Class-level patterns have the largest span of any of the implementation patterns. Design patterns, by contrast, generally talk about the relationships between classes.

The following patterns appear in this chapter:

• Class
  Use a class to say, “This data goes together and this logic goes with it.”
• Simple Superclass Name
  Name the roots of class hierarchies with simple names drawn from the same metaphor.
• Qualified Subclass Name
  Name subclasses to communicate the similarities and differences with a superclass.
• Abstract Interface
  Separate the interface from the implementation.
• Interface
  Specify an abstract interface which doesn’t change often with a Java interface.
• Abstract Class
  Specify an abstract interface which will likely change with an abstract class.
• Versioned Interface
  Extend interfaces safely by introducing a new sub-interface.
• Value Object
  Write an object that acts like a mathematical value.
• Specialization
  Clearly express the similarities and differences of related computations.
• Subclass
  Express one-dimensional variation with a subclass.
• Implementor
  Override a method to express a variant of a computation.
• Inner Class
  Bundle locally useful code in a private class.
• Instance-specific Behavior
  Vary logic by instance.
• Conditional
  Vary logic by explicit conditionals.
• Delegation
  Vary logic by delegating to one of several types of objects.
• Pluggable Selector
  Vary logic by reflectively executing a method.
• Anonymous Inner Class
  Vary logic by overriding one or two methods right in the method that is creating a new object.
• Library Class
  Represent a bundle of functionality that doesn’t fit into any object as a set of static methods.